The present invention relates to a coated cutting tool for metal machining, having a substrate of a hard alloy and, on the surface of said substrate, a hard and wear resistant refractory coating is deposited by Physical Vapor Deposition (PVD). The coating is adherently bonded to the substrate and is composed of a composite layer having at least two phases, the layer being under compressive stress. The layer comprises a metal oxide/oxide composite consisting of two components with different composition and different structure. The composite layer grain size is in the nanometer scale.
The process of depositing thin ceramic coatings (from about 1 to about 20 μm) of materials like alumina, titanium carbides and/or nitrides onto, e.g., a cemented carbide cutting tool is a well established technology and the tool life of the coated cutting tool, when used in metal machining, is considerably prolonged. The prolonged service life of the tool may under certain conditions extend up to several hundred percent greater than that of an uncoated cutting tool. These ceramic coatings generally comprise either a single layer or a combination of layers. Modern commercial cutting tools are characterized by a plurality of layer combinations with double or multilayer structures. The total coating thickness varies between from about 1 to about 20 μm and the thickness of the individual sublayers varies between a few micrometers down to some hundredths of a micrometer.
The established technologies for depositing such layers are CVD and PVD (see, e.g., U.S. Pat. Nos. 4,619,866 and 4,346,123). PVD coated commercial cutting tools of cemented carbides or high speed steels usually have a single layer of TiN, Ti(C,N) or (Ti,Al)N, homogeneous in composition, or multilayer coatings of said phases, each layer being a one phase material.
There exist several PVD techniques capable of producing thin, refractory coatings on cutting tools. The most established methods are ion plating, magnetron sputtering, arc discharge evaporation and IBAD (Ion Beam Assisted Deposition) as well as hybrid processes of the mentioned methods. Each method has its own merits and the intrinsic properties of the produced layers such as microstructure and grain size, hardness, state of stress, cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool being used in a specific machining operation can thus be accomplished by optimizing one or several of the above mentioned properties.
Particle strengthened ceramics are well known as construction materials in bulk form, however, not as nanocomposites until recently. Alumina bulk ceramics with different nanodispersed particles are disclosed in J. F. Kuntz et al, MRS Bulletin January 2004, pp 22-27. Zirconia and titania toughened alumina CVD layers are disclosed in for example U.S. Pat. Nos. 6,660,371, 4,702,907 and 4,701,384. In these latter disclosures, the layers are deposited by CVD technique and hence the ZrO2 phase formed is the thermodynamically stable phase, namely the monoclinic phase. Furthermore, the CVD deposited layers are in general under tensile stress or low level compressive stress, whereas PVD layers are typically under high level compressive stress due to the inherent nature of the PVD process. In DE 10251404, blasting of alumina/zirconia CVD layers is described to give a compressive stress level. Blasting processes are known to introduce compressive stresses at moderate levels.
Metastable phases of zirconia, such as the tetragonal or cubic phases, have been shown to further enhance bulk ceramics through a mechanism known as transformation toughening (Hannink et al, J. Am. Ceram. Soc 83 (3) 461-87; Evans, Am. Ceram. Soc. 73 (2) 187-206 (1990)). Such metastable phases have been shown to be promoted by adding stabilizing elements such as Y or Ce or by the presence of an oxygen deficient environment, such as vacuum (Tomaszewski et al, J. Mater. Sci. Lett 7 (1988) 778-80), which is typically required for PVD applications. Variation of PVD process parameters has been shown to cause variations in the oxygen stoichiometry and the formation of metastable phases in zirconia, particularly the cubic zirconia phase (Ben Amor et al, Mater. Sci. Eng. B57 (1998) 28).